Generally, in order to drive the air-conditioning fan or the pump circulating the water, the torque of the driving motor has to be increased in proportion to the square of the speed. In this way, when the power supply voltage is directly connected to the motor driving the load of the quadratic torque type, energy waste is increased. In other words, if a power frequency of 60 Hz is used as a reference of the rotation speed and an induction motor having 4 poles is taken as an example, the reference speed becomes 1800 rpm, and the speed is operated regardless of the load condition. Therefore, it has been causing unnecessary energy consumption under partial load conditions. This problem can be solved by applying an inverter that controls the speed according to the load.
The load and the energy saving principle are explained more specifically.
As shown in FIG. 1, a load, for example, quadratic torque load, is a load of a centrifugal pump and a centrifugal fan, and is characterized in that electric power is consumed in proportion to the three squared of speed due to friction and inertia. Therefore, reducing the speed of the motor reduces power consumption further, which can significantly reduce energy.
To explain further, suppose that the pump is operated at a speed of 1800 rpm for one hour. At this time, it is assumed that the consumed power or electric power charge is 100%. Then reduce the pump by 50% at a speed of 900 rpm, and instead increase the operating time by 2 times to 2 hours. Then, the actual amount of flow pumped by the pump is the same, but its power usage or electricity charge is only 14% instead of 100%. In other words, by reducing the speed by 50%, the power consumption can be saved by as much as 86%.
If this pump is 37 kW, if we assume that 37 kW will be operated one hour a day for one year, that electricity fee will be charged around 1195$. However, to operate one hour a day for 4 hours with different speeds, instead of reducing the speed, its power is estimated to be around 19$. Also, if you run the pump slowly in 6 hours, the power is charged by 48$, but you can supply an amount like 1,194$ electricity before.
As described above, because the power is consumed in proportion to the three squared power of the speed, the power reduction can be dramatically reduced if the speed is reduced. The power consumption proportional to the three squared power is called affinity law #3.Therefore, energy can be saved by controlling the motor by using the inverter.
Hereinafter, the control operation according to the load size of the conventional inverter technology will be described in detail.
First, the characteristics of a motor, that is, an induction motor will be described.
The speed N (r/min) of the induction motor is expressed by the following formula.
                    N        =                                            120              ×              f                        P                    ⁢                      (                          1              -              S                        )                                              [                  formula          ⁢                                          ⁢          1                ]            
Where N is the speed of the induction motor (or motor), f is the frequency, P is the number of motor poles, and S is the slip.
According to affinity law #1, the flow rate Q is proportional to the change in the number of revolutions (or the speed of the induction motor) (N). Therefore, the faster the speed of the induction motor, the more the flow rate supplied by the motor increases proportionally.
In addition, there is the following relationship for the voltage V of the induction motor, the frequency f and the magnetic flux (torque).
                    Φ        ∝                  V          f                                    [                  formula          ⁢                                          ⁢          2                ]            
Where Φ is the magnetic flux, V is the voltage, and f is the frequency.
At this time, the magnetic flux (Φ) or torque (τ) of the induction motor should be controlled by the size of the load, that is, the size (or height) of the head H. That is, It is necessary to control the voltage applied to the motor to the minimum magnitude that the maintaining and torque flux drive the load size.
Since the inverter can arbitrarily control the frequency f and the voltage V, the induction motor can be operated at a variable speed.
As an example of pumping, the flow quantities and heads at the load side correspond to the frequency f and the voltage V at the driver (inverter and induction motor) side, respectively. That is, by the inverter, how fast and how much power will be pumped up. That is, the variable that determines how fast the pump will pumped up is the frequency f, and the water head that determines how high the pump is raised determines the voltage V at the inverter side. Specifically, when pumping up the water, the element of “how much flow, how fast will you pumped up the water”, the voltage factor is determined by the location of the reservoir tank, such as 5, 10, or 60 stories high. The control element of the pump is “how fast” and “how high” it is to be raised, the former corresponds to the flow Quantity, and the latter corresponds to the head. The higher the water head, the higher the water pressure, so the bigger the torque is required in the pump to meet the water pressure, so the “head/pressure/torque” has the same meaning.
So, in order to reduce the power to the maximum, the voltage corresponding to the water head must be controlled, and the power can be reduced to the maximum by selecting the lowest frequency. Conversely, reducing power to the maximum level results in the highest efficiency.
However, according to the related technology, when the motor for the load is controlled by the inverter, mainly the speed of the motor is controlled[Patent Literatures 1 and 2].
That is, the inverter of the related technology is used to determine the number of revolutions of how fast the motor is to be rotated, and the inverter determines the number of revolutions of the motor. Then, the motor manufacturer provides the water head and flow rate curve of the corresponding motor. This water head and flow curve, that is, the H-Q curve (water head-flow curve) are made with the curve required for inverter control using affinity's first law and second law, and accordingly H-N curve (water head-flow curve) to control the inverter.
In other words, as shown in FIG. 2, the flow rate Q can be obtained by applying H-N curve to the motor (or inverter) using the H-N curve after applying the affinity law #1, #2 of the following formulas 3 and 4.Q∝N   [formula 3]H∝N2(T∝N2)   [formula 4]
Where Q is the flow rate (or air flow rate), N is the velocity, τ is the torque, and H is the head (or pressure, head, static pressure).
As a result, the current technology controls the inverter to match the H-N curve. Therefore, if the head H is high, the speed N must be controlled to be high. For example, suppose a situation in which an inverter is required to operate at 1800 rpm. At this time, suppose that the necessary head (H) is 120 Pa. It is also assumed that the flow rate required by the current system is 1200 rpm in order to reduce power by the affinity low. That is, the speed of the motor can be reduced from 1800 rpm to 1200 rpm. However, when I tried to rotate the motor speed to 1200 rpm, when I looked at 1200 rpm, the head (H) was only 50 Pa. Therefore, since the required head is 120 Pa, it can not be pumped up when operated at 1200 rpm. However, according to the H-N curve, when the head is 120 Pa, the motor speed must be 1600 rpm.
Therefore, the motor speed can be reduced from 1800 rpm to 1200 rpm for power reduction, but it can be reduced to only 1600 rpm due to the H-N curve limitation. As described above, power consumption is proportional to the three squared of the speed by the affinity low. According to the related technology, power can be saved only by the third power of 200 rpm at 1800 rpm and 1600 rpm. However, since the motor speed can be reduced to 1200 rpm, the power corresponding to the third power of 400 rpm, which is the difference between 1600 and 1200, is wasted.
When the motor is rated at an efficiency in rated operation, the motor becomes saturated by increasing the voltage with respect to the rated frequency in EMF equation V=4.44fnφ(V: voltage, n=number of turns, φ=magnetic flux). Therefore, the frequency control of inverter controls the ratio of rated voltage/rated frequency constantly.
As mentioned earlier, in order to raise the water, it is necessary to control both the flow rate and the head of water. However, the head of water does not fit properly when the control is performed only by the flow rate. Thus, according to the related technology the two control elements, namely, the flow rate and the head of water, must be satisfied, so that the water is quickly pumped up and the flow rate is greatly increased to match the head of water. As a result, 400 rpm is consumed more and power corresponding to three squares of 400 rpm is wasted. Calculating this, it is estimated that 44% of the power is wasted. Therefore, it can be seen that the conventional inverter is not able to control the voltage, so that the power is wasted at a large amount in the square-root reduction load. Since the conventional inverter can not control the voltage, it is necessary to raise the water at a speed of 1200 rpm. However, the water is pumped at a speed of 1600 rpm, which causes a large amount of power waste due to the affinity law.
Further, techniques using an inverter for controlling the torque have been proposed (Patent Documents 3 and 4), and vector control inverters for torque control have been commercialized. Therefore, it can be applied to the load by using an inverter that controls the torque instead of the voltage. However, in the case of an inverter using a torque, that is, a vector-controlled inverter, the inverter is sold at a price about 2.5 times as high as that of an inverter that controls only a speed. In addition, even a vector-type inverter capable of controlling torque is not designed to achieve maximum efficiency control in a load. Even if it is designed with maximum efficiency control, it is impossible to control at the maximum efficiency practically because all the necessary conditions in the load are not known by the inverter.
In other words, the vector control (Vector control, Field oriented control) method separates and controls the torque component and the flux component, and is widely used for high performance variable speed control with excellent control performance. However, since the number of controllers is large and complex and the amount of information to be detected is large, the manufacturing cost is high and it is disadvantageous in terms of economy.
In summary, in the related technology, when the voltage depending on the flow rate is larger than the required water head, it can not be reduced to the voltage suited to the water head, so that it is impossible to reduce the power consumption, so there is a problem that the maximum efficiency operation can not be performed.